Synchronizing apparatus



March 15, 1938. E. 1-1. VEDDER SYNCHRONI Z ING APPARATUS 3 Sheets-Sheet 1 Original Filed Feb '7, 1934 INVENTOR [did/)7 H. VC/dl".

ATTOk RN WlTN ESSES:

March 15, 1938. VEDDER 2,111,014

SYNCHRONI Z ING APPARATUS Original Filed Feb. 7, 1934 3 Sheets-Sheet 2 INVENTOR Edwm l edder.

ATTORNEY March 15, 1938. H, VEDDER 2,111,014

SYNCHRONI Z ING APPARATUS Original Filed Feb. '7, 1934 3 Sheets-Sheet 3 WITNESSES: I INVENTOR WMJM [aw/'27 H. Veclar 7 BY I Patented Mar. 15,, 1938 UNITED STATES PATENT OFFICE SYNCHRONIZING APPARATUS Pennsylvania Original application February 7, 1934, Serial No. 710,095. Divided and this application May 8,

1936, Serial No. 78,597

Claims.

My invention relates to synchronizing apparatus and has particular relation to synchronizing systems of the type that have particular utility in installations where current is to be supplied in accurately timed periodic pulses as is the case in line and spot welding with alternating current.

This application is a division of application Serial No. 710,095, filed February 7, 1934.

In a system for controlling the flow of alternating current to a resistance welding circuit, it is desirable to provide an inertialess means for satisfactorily effecting the control of the cur rent flow. Such means is desirable in order to provide an efiicient, economical and accurate means whereby the alternating current supplied to the resistance welding circuit may be adjusted to the desired values and maintained for predetermined time intervals with an unvarying degree of accuracy over a long period of operation.

In the past, switches of various types have been used for controlling the current flow to a resistance welding circuit, all of which have been open to the objection that a certain amount of sparking or arcing occurs when the control circuit is completed or broken. The sparking or arcing is undesirable principally for the reason that it causes increased wear on the contact members and results in materially decreasing their life as well as the accuracy with which the control is effected. Further, the arcing may cause an explosion in the event that an explosive gaseous atmosphere exists in the vicinity of the contact members.

It is well known that it is highly desirable to provide a very accurate timing of the duration of the current flow in a resistance welding circuit, as well as to control the instant of time when the current is applied, in order to prevent burning of the material or to provide sufficient current to perform the welding operation and to prevent surges in the power system supplying current to the welding circuit. If the current is applied for too long a time, particularly on relatively thin materials, they are likely to be burned or destroyed. If sufllcient current is not applied a poor weld ordinarily results. If the current is not applied at the proper instant in the current cycle, transient effects will be present, as is well known, due to the magnetic characteristics of the transformers which are connected to the I welding circuit.

crating current which shall be simple and emcient in operation, and which may be readily and economically manufactured and installed.

The principal object of my invention is to provide for accurately controlling the instant of application and duration of application of alternating current flow in a circuit such as a welding circuit by suitably synchronizing the application of the current with the alternating current supply source.

Another important object of my invention is to provide inertialess and sparkless meansfor controlling the current flow in a resistance welding circuit or the like.

Another object of my invention is to provide for applying welding current or current for an analogous purpose at the zero point of the current wave.

Still another object of my invention is to provide for changing the average value of welding current and thereby the welding heat by changing the time during which current is conducted during each half cycle.

A further object of my invention is to provide for adjusting a welding system according to the power factor of the welding circuit and for shifting a scale to correspond to the power factor of the circuit for indicating the required settings to provide different welding heats.

Other objects of my invention will, in part, be obvious and, in part, appear hereinafter.

My invention accordingly is disclosed in the embodiment hereof shown in the accompanying drawings and comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth and the scope of the application of which will be indicated in the appended claims.

For a more complete understanding of the nature and scope of my invention, reference may be had to the following detailed description taken in connection with the accompanying drawings,

in which: t

Figure 1 is a top plan view of a timer which may be used in practicing my invention;

Fig. 2 is a view, in side elevation, of the timer shown in Fig. 1;

Fig. 3 is a view, in end elevation of a portion of the timer shown in Fig. 1, showing an adjustable scale which may be used for setting the timer;

Fig. 4 is a detail plan view of the timer disc;

Figs. 5 and 6 are detail plan views showing the assembly of the timer disc with diil'erent blanking discs;

Fig. '7 illustrates diagrammatically the circuit connections which may be used in one embodiment of my invention; and

Figs. 8 and 9 show a number of curves which demonstrate certain operating characteristics of the welding circuit.

Referring now particularly to Figs. 1 and 2 of the drawings, the reference character I0 designates generally, a synchronous motor which is arranged to drive a rotatably-mounted plate II through a set of reduction gears (not shown) which may be contained within a housing l2. The synchronous motor III is disposed to rotate the plate II through the reduction gears at such a rate that the plate I I will be rotated once each second, or sixty times a minute.

A main disc I3 is a mounted on the plate II and is fixedly positioned thereon by means of dowel pins I4 and a thumb nut I5. It will be understood that the dowel pins I4 are fixedly mounted on the plate II and that they extend into suitable apertures in the main disc I3.

The periphery of the main disc I3, as is more clearly shown in Fig. 4 of the drawings, is provided with a plurality of teeth I6 and slots II. In this instance the main disc I3 is provided with 120 teeth l6 and slots II. Thus, when the plate II is rotated at the speed of one revolution per second, 120 teeth I6 or slots II will pass each second a stationary point located near the perlphery of the main disc I3 or, in point of time, there will be one tooth IE or one slot I'I per halfcycle, if a sixty-cycle current source is used.

When a sixty-cycle power system is used for operating a welding system, with which this apparatus is herein illustrated as employed, it will be apparent that each of the teeth I6 or slots I'I corresponds to one-half cycle of the alternating current in point of time. It will be readily apparent, however, that other speeds of rotation of the main disc I3 may be employed as well as other teeth and slot combinations in the periphery thereof without departing from the scope of my invention. It will also be apparent that other types of openings may be provided in the disc I3 such as circular or rectangular apertures positioned at the desired spaced intervals.

A photo-electric cell I8 is provided underneath the main disc I3 and above it is positioned a light source I9 provided with a lens 20, located in a lens housing 2I. The lens 20 is provided for focusing the rays of the light source I 9 on the photo-electric cell I8. An arm 22, provided with an aperture 23, is positioned to more sharply define the rays of light as they are applied to the photo-electric cell I8. It will be observed that the rays of light from the light source I9 will be intercepted by the teeth I6 of the main disc I3 and that the rays of light will be permitted to impinge upon the photo-electric cell I8 when any of the slots II are positioned therebetween.

It is desirable to position the lens 20 with respect to the main disc I3 so that its focal point is located in the plane of the main disc I3. With this arrangement any tendency of the main disc I} to be distorted from its stationary horizontal position during rotation will not materially affect the instant that the light rays are applied to the photo-electric cell III, as the teeth I6 intercept the light rays.

In the event that rays of light are arranged in the form of a cone, the apex of which is below the disc I3, there may be some variation in the timing due to distortion of the disc II. This variation would be caused by the teeth I8 uncovering the light rays at different points along the surface of the cone. This effect may be obviated by tilting the photo-electric cell It and the light source I9, together with the associated parts, to such a degree that the trailing edges of the teeth I6 will be parallel to the surface of the cone of light rays on the side where they are first uncovered.

As illustrated in the drawings, the photo-electric cell I8 and the light source I9 are mounted on a bracket 24 which is positioned to rotate around the vertical axis of the plate I I and main disc I3. In order to alter the time of application of the light rays to the photo-electric cell ll,

corresponding to various points on the voltage or current wave, the bracket 24 may be rotated slightly to effect the desired timing. An arm 25, secured to the bracket 24, is positioned between a pair of adjusting screws 26 and 21 which are threadably mounted in a frame 28. It will be understood thatthe position of the arm 25, and thereby the position of the bracket 24, may be altered by turning the adjusting screws 26 and 21 in the proper direction to effect the desired change in the timing. It will also be understood that the timing may be adjusted by rotating the frame of the motor I0 instead of moving the bracket 24 and the apparatus carried thereby.

In view of the fact that the power factor of the welding circuit may be different for differ ent materials, the position of the arm 25 will be different for the different power factors when the maximum heating is applied to the welding circuit. In order to provide for indicating the adjustment for different heating values for the different power factors an adjustable scale 28 may be used which is slidable on the frame 28. As illustrated, the scale 29 is provided with a slotted opening 30 with which a thumb screw 3| is arranged to engage and secure the scale 20 in any desired position. The arm 25 may be provided with a pointer 32 and the scale 29 may be provided with graduations 33 to which the pointer 32 may be adjusted to provide corresponding degrees of welding heat with reference to the maximum heat which is available.

In the event that the main disc I! only is employed, the alternating current will be supplied to the welding circuit during each succeeding half-cycle. While such operation may be desirable under certain circumstances, it is more often the case that it is desired to apply the welding current for a predetermined number of halfcycles and, then, to prevent the application of welding current for a predetermined number of half-cycles which may or may not be the same as the number of half-cycles during which the current is applied.

In order to apply the welding current for two cycles and to prevent its being applied for two cycles, every other four of the slots I'I may be blanked out by means of an auxiliary or blanking disc which, as is shown more clearly in Fig. 5 of the drawings, is provided with a notched periphery that is arranged to cover alternate groups of four of the slots II. A plate 36, secured to the main disc I3 by means of screws I1, is arranged to hold the auxiliary disc 35 in position.

In order to obtain a different combination of current cycles as applied to the welding circuit, the auxiliary disc 38, shown in Fig. 6 of the drawings, may be provided. The disc 3| is arated and is then filled with mercury vapor.

ranged to cover four successive slots l1 and to leave open the next two slots ll. With this arrangement, alternating current will be applied to the welding circuit for one cycle and will not be applied for two cycles.

I have found that the auxiliary discs and 38 may be formed of such material as paper, although, of course, other material such as aluminum, copper or the like may be used. By providing several auxiliary discs such as 35 and 38, it is possible to provide a relatively unskilled operator with means for accurately and readily obtaining a large combination of times of application of welding current with respect to the time during which it is not applied. It is only necessary for the operator to remove one set of the timing discs from the plate- I l. and to position thereon the desired set which will provide the desired combination of time during which the welding current is applied and of time during which the welding current is not permitted to flow in the welding circuit. It is pointed out, however, that the accuracy of the timing does not depend on the accuracy of the auxiliary disc 35, but rather that it depends upon the main disc l3, which may be very accurately cut or milled.

The timing device described hereinbefore may be used for controlling the operation of the welding system shown in detail in the diagram in Fig. 7 of the drawings. Referring now particularly to Fig. 7 of the drawings, it will be observed that the reference character 48 designates, generally, a welding transformer having a primary winding 4| and a secondary winding 42. The secondary winding 42 may be connected to a welding circuit which, in this instance, comprises a pair of roller welding electrodes 43 between which work 44, on which a welding operation is to be performed, may be positioned. It will be understood, however, that the secondary winding 42 may be connected to other types of welding electrodes such as those used for spot welding, rather than the roller electrodes .43 which are customarily used for line welding. The primary winding 4| of the transformer 48 may be connected to the secondary winding 45. of a power transformer, shown generally at 46, the primary winding 41 of which may be connected to a suitable source of alternating current 48, such, for example, as a fill-cycle source.

In order to vary the current flow to the welding transformer 40, an adjustable tap 49 may be provided along the primary winding 4| of the welding transformer 46. However, as will be set forth in detail hereinafter, it is unnecessary to provide the adjustable tap 49 in the event that advantage is taken of certain adjustments which have been previously described for controlling the time of application of the light rays from the light source l9 to the photo-electric cell l8.

In order to control the application of alternating current to the welding transformer 49, vaporelectric devices, shown generally at 58 and 59 are provided. The devices 58 and 59 are of the mercury vapor type, in each of which an anode 68 is provided, as illustrated, in a suitable container 6|, at the bottom of which is located a pool of mercury 62. The container Si is evacu- In order to render the vapor-electric devices 58 and 59 conducting, a starting electrode or crystal 63' is provided in each of them, which comprises a material such as boron, carborundum or the like.

While two individual vapor-electric devices 58 and 59 are illustrated, it will be readily understood that a single vapor-electric device having two mercury pools may be used, in which the mercury pools alternately function as anode and cathode. It has been found that, when a certain amount of current is caused to flow through'the starting electrodes 63 into the mercury pools 62, cathode spots are formed which cause the mercury vapor within the containers 6| to become conducting, provided the proper polarities are applied to the anodes 68 and the mercury pools 82, which form the cathodes. Thus, it is only necessary to pass sufllcient starting current through the starting electrodes 63 to form the cathode spots at a predetermined time in the proper half -cycle in the alternating-current wave to render the vapor-electric devices 58 and 59 conducting for the remainder of their particular half-cycles.

In order to conduct succeeding half-cycles of alternating-current, the vapor-electric devices 58 and 59 are connected inversely, that is, the anode 60 and the cathode 62 of the devices 58 and 59, respectively, are connected together while the cathode 62 and the anode thereof are connected together, as illustrated in the drawings. It will, therefore, be evident that, if the vaporelectric devices 58 and 59 are rendered conducting by energizing the starting electrodes 63, current will continue to flow therethrough as long as this condition is maintained. However, if the starting electrodes 63 are not energized during a time interval corresponding to a time longer than the half-cycle during which either of the vapor-electric devices 58 or 59 are adapted to be conducting, neither of them will conduct a succeeding half-cycle. In other words, it is neces-, sary to ignite the vapor in the devices 58 and 59 for each half-cycle for which they are adapted to be conducting and if they are not so ignited, then they will not conduct for periods longer than one-half cycle.

In order to render the vapor-electric devices 58 and 59 conducting, control tubes 64 and 65, of the hot cathode. type, may be provided and their anodes 66 and 61 may be connected to the anodes of the vapor-electric devices 58 and 59, respectively, as shown. The cathodes 68 and 69 of the control tubes 64 and 65 are connected in the customary manner to the starting electrodes 63 of the vapor-electric devices 58 and 59, respectively. The control tubes 64 and 65 are arranged to pass suflicient current through the starting electrodes 63 so that the cathode spots will be formed, as set forth hereinbefore, to render the vapor-electric devices 58 and 59 conducting.

In order to initiate the functioning of the control tubes 64 and 65 a timer tube 10 is provided which is arranged to apply the output of the photo-electric cell I8, as amplified by an amplifier 'Il through an insulating or impulse transformer 12 to a resistor 13. The control impulse thus obtained is of such a polarity as to overcome the negative potential which is normally applied to the grids l4 and 15 of the control tubes 64 and 65 by means of a direct-current source, such as a battery 16. The polarity of the control impulse is indicated by the customary polarity signs located at the ends of the resistor 13. The insulating or impulse transformer 12 also serves for the purpose of insulating the control circuit including the timer tube 10 and the amplifier II from the main or welding circuit in order that the former circuit may not be affected by different grounding conditions in the latter circuit.

In order to control both of the grids l4 and II of the control tubes 84 and 85 from a single control circuit, a transformer I1 is provided having a primary winding I8 connected to the source 48 of alternating-current and a secondary winding 18 connected to the grids l4 and 15, as illustrated. In addition, a resistor 88 is connected between the anodes 88 of the vapor electric devices 58 and 58. By means of the transformer 11 and the resistor 88, th'e potentials of the cathodes 88 and 89 of the control tubes 84 and 85 are reduced to a common point with respect to the control potentials which are applied thereto by means of the battery 18 or as obtained through the impulse transformer I2. It will be observed that the control potentials, as obtained from the battery 18 and the impulse transformer 12, are applied to the midpoints of the secondary winding I8 and the resistor 88. It is, therefore, unnecessary to provide a control circuit individual to each of the control tubes 84 and 88, although the anodes and cathodes of these tubes are oppositely connected.

It wil be observed that the plate potentials for the timer tube 10 and the amplifier Il may be obtained from a plate rectifier, shown generally at 82, of the copper-oxide type, which may be energized from a suitable plate transformer 88. In like manner, the grids of the timer tube Ill, and the amplifier 'Il, may have the proper potential applied thereto by means of a grid rectifier shown generally at 84, which may also be of the copperoxide type and it may be energized by means of a transformer shown generally at 85. A filament transformer, shown generally at 88, is provided for applying the proper heating current to the filaments of the timer tube 18 and amplifier Ii. Since the operation and functioning of the timer tube III, as controlled by the amplifier I I are well known to those skilled in the art, adetailed description thereof will not be set forth in this speciflcation.

It will be observed that the synchronous motor i8, which is employed for driving the main disc I8, is arranged to be energized from the source 48 of alternating current. These connections are desirable in order to synchronize the operation of the control system with the frequency of the source 48 of alternating-current so that the desired control of the flow of current to the welding circuit may be effected.

With a view to measuring the direct-current component of the alternating current which may exist in the welding circuit under certain conditions there is provided a meter 9i, such as a direct-current milli-volt meter, connected across a resistance shunt 80 in the supply circuit. Since the direct current, resulting from certain transient conditions, may flow in either direction, the meter 8| is arranged to have a scale with a zero center. It will be apparent that the meter 9i may then be caused to indicate the presence of direct current, regardless of the direction of fiow of such direct current.

A manually-operable control switch 82 is provided in the circuit connecting the impulse transformer 12 to the resistor 18 in order to permit the operator to control the functioning of the system as desired. It will be understood that the control switch 82 may be of any suitable type such as a push button switch or a foot-operated switch.

In operation, it will be assumed that the source 48 of alternating current is energized to apply a -cycle frequency to the power transformer 48. It will also be assumed that the synchronous motor i8 is operating to rotate the main disc i8 at a speed of one revolution per second, and for that, the disc combination, shown in Fig. 5 is provided. It will be further assumed that the proper energizing potentials are applied to the various elements of the control tubes 84 and 85, the timer tube I8 and the amplifier II and that the control switch 82 is closed.

Under the aforesaid conditions, current will be applied to the welding electrodes 48 for two cycles and it will not be applied for the next two succeeding cycles. This condition is illustrated by the curves shown in Fig. 8 of the drawings, the curve E representing the voltage of the welding circuit and the curve I representing the current. The phase displacement between these curves represents the power factor of the welding circuit which, as will be readily understood, will vary depending upon the material which is positioned between the welding electrodes 48. The full lines representing the curves E and I illustrate the time during which the vapor-electric devices 58 and 59 are ordinarily rendered conducting, while the dotted lines represent the time during which they are not ordinarily conducting.

As has been set forth hereinbefore, it is necessary in this embodiment of my invention to render the vapor-electric devices 88 and 58 conducting for each half-cycle. Thus, as the control electrode of the vapor-electric device 58 is energized, at a time corresponding to T1, it will become conducting at that instant and will remain in this state until the end of this particular half-cycle. However, it is necessary to again apply the energizing potential to the control electrode 58 at a time T2 in the next succeeding half-cycle for which the vapor-electric device 58 may be adapted to be conducting in order to cause current to flow therethrough. In like manner, it is necessary to energize the control electrode 68 of the vapor-electric device 58 at times '1; and T4 in order to render it conducting for its corresponding half-cycles. It will be understood that, at each instant when one of the slots i1 is positioned between the light source I! and the photo-cell l8, times corresponding to T1, T2, T3 and T4 occur, at which instants the vaporelectric devices 58 and 59 are caused to be conducting.

In the event that the vapor-eiectric device 58, for instance, should be rendered conducting at a time corresponding to T5, when normally the next instant, when it should be rendered conducting, would occur at or after the next zero point of the current wave I, a transient phenomenon will appear which, as will be readily understood, will be due to the magnetic condition of the transformers connected to the welding circuit. As a result, the current in the half cycle will'not follow the curve I but will follow a curve which may be represented by the dotted line I for one half-cycle which will represent a current of considerably greater magnitude than would ordinarily be present. This transient phenomenon causes a certain amount of direct current to flow in the circuit which may be indicated by the meter 8!. These surges will be reflected in the source 48 of alternating current and will cause overloads to be applied thereto which are undesirable. Further, due to the application of a direct current component in the voltage applied to the primary winding 4| of the welding transformer 48 and to the secondary winding 45 of the power transformer 48, the magnetizing currents for the transformers will be increased, thereby increasing the amount of current reill quired to be conducted by the vapor-electric devices 58 and 59 beyond that which they may be capable of conducting. It is, therefore, desirable to reduce this transient effect so that no direct current will flow in the circuit.

This correction may be obtained by causing the vapor-electric devices 58 and 59 to become conducting ata time at or after the next succeeding zero point of the current curve has been reached. Thus, if the vapor-electric device 58, for example, is rendered conducting at a time T6 or at any tine alter this instant in the next succeeding half-cycle, this transient phenomenon will not occur. Since the time T6 will vary, depending upon the power factor of the circuit, it is desirable to be able to adjust the position of the photo-electric cell l8 and the light source I!) for each different power factor. This adjustment may be made by loosening the thumb screw 3| which holds the scale 29 in position. The position of the photo-electric cell I8 and the light source I9 is then adjusted by means of the screws 26 and 21 until the meter 9| reads zero, thereby indicating that no direct current flows in the welding circuit. The scale 29 may then be adjusted so that the graduation corresponding to welding electrodes 43, since the vapor-electric devices 58 and 59 will be conducting for complete half-cycles. In the event that it is desired to reduce the heating below the maximum,

it is merely necessary to adjust the position of the photo-electric cell l8 and the light source l9 by means of the screws 26 and 21 in the direction of rotation of the main disc l3, as indicated by the arrow thereon in Fig. 1, to a predetermined point along the graduations 33 as may be desired. Thus, instead of the vapor-electric device 58, for instance, being caused to become conducting at the time T6, it may be caused to become conducting at the time T1. It will be observed that the shaded area under the curve 1. representing the current which is supplied to the welding electrodes 43, represents the total quantity of current which is applied. If this area is increased or decreased the average value of the current will be correspondingly increased or decreased, and, as a result the heating effect, as available at the Welding electrodes 43, will be correspondingly altered. For the sake of clarity the current curve I, representing the conducting condition initiated at the time T1 has been shown as being identical with the shape of the current wave resulting from initiating the conduction at the time T6 while, in fact, it would be slightly different.

It will be apparent that it will not be necessary to provide the adjustable tap 49 on the primary winding 4| of the welding transformer 48 in order to vary the amount of current which is applied for performing the welding operation. This complication and expense may be obviated by the simple expedient of adjusting the times at which the vapor-electric devices 58 and 59 are arranged to become conducting in their corresponding half-cycles and thereby changing the average value of the current which is provided for performing the welding operation.

The curves illustrated in Fig. 9 of the drawings represent the voltage and current conditions in the welding circuit which occur when the disc combination, shown in Fig. 6, is used. With this arrangement, current is applied for one cycle as indicated by the full-line curves and is not applied for two cycles, as illustrated by the dotted lines. It will be observed that the power factor of the welding circuit, as represented by the curves shown in Fig. 9, is considerably less than the power factor of the welding circuit as represented by the curves shown in Fig. 8. This change in the power factor, which may occur under different conditions, illustrates the necessity for providing an adjustment to compensate for changes in power factor.

The shaded area under the current curve I in Fig. 9 represents the amount of current which is applied for performing the welding operation. The particular current values illustrated may be obtained by adjusting the position of the photoelectric cell and the light source l9, as set forth hereinbefore, to obtain the relatively small current value as indicated.

Since certain further changes may be made in the above construction and different embodiments of the invention may be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be inwrpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. Apparatus for supplying current to weld material from an alternating source comprising valve means in circuit with said source for controlling the flow of welding current to said material, means for restraining said valve means from being conductive except during a predetermined number of half-cycles to time the supply of welding current to said material and means for restraining said valve means from being conductive during a predetermined portion of each of said half-cycles to control the supply of heating energy to said material. I

2. Apparatus for supplying current to weld material from an alternating source comprising valve means in circuit with said source for controlling the flow of welding current to said material, means for restraining said valve means from being conductive except during a predetermined number of half-cycles to time the supply of weld ing current to said material and means adjustable independently of the number of half-cycles during which said valves are conductive for restraining said valve meansiirom being conductive during a predetermined portion of each of said half cycles to control the supply of heating energy to said material.

3. The method of resistance welding material from a source of periodically pulsating potential which comprises the steps of timing the supply of welding current to said material by supplying current sufficient to weld the material only during a number of periods of said source that is predeterminable at will to correspond to the properties of said material and controlling the supply of heat to weld said material by initiating the supply of current during each of the lastmentioned periods of said source at instants in the said periods that are predeterminable at will to correspond to the properties of said material.

4. The method of resistance welding material from a source of periodically pulsating potential which comprises the steps of timing the supply of welding current to said material by supplying, in synchronism with said source, current sumwhich comprises the steps of timing the supply of welding current to said material by supplying, in

synchronism with said source, current sumcient to weld the material only during a number 01' periods of said source that is predeterminable at will to correspond to the properties of said material and controlling the supply of heat to weld said material by initiating, in synchronism with said source, the supply of current during each of the last-mentioned periods of said source at instants in the said periods that are predeterminable at will to correspond to the properties of said 10 material.

EDWIN H. VEDDER. 

